Flowmeter for purge

[Kdolph]

Hi, I attached a diagram of the situation.

http://files.engineering.com/getfile.aspx?folder=f81381a2-ffd0-4dc4-8347-7344f9452c79&file=Purge.pdf

We want to measure the flowrate of the purge of an evaporator. This works as a distiller (it evaporates water), so the purge is just water with high levels of conductivity.
We want a purge of 20 m3/hr. This purge is discarted as it is seen in the isometric view, through a 3” pipe. IN the water/steam separator of the evaporator the average pressure/temperature are 2,5 bara and 127,4°C so I assume this is the condition at the exit of the purge. This purge go to an vertical 8” pipe that works as a collector and due to the pressure reduction some steam flashes out the collector.
So if I want to measure the flowrate of the purge. Could I do it in the next way:
Installing an magnetic flowmeter downstream the collector pipe for measuring the water part of the purge and estimate the flashing with some thermodynamic correlation? I think it is not possible to measure the flowrate upstream the pipe collector because of the the closeness of the pressure vapour with the system pressure.

 

[danw2]

1) Magmeters hate bubbles and truly want a 'full pipe', meaning all liquid. If there's flashing inside the magtube or in the water carried into the magtube the magmeter readings bounce all over the place and are useless.

2) There are mag tubes rated for higher tempeatures and there are tubes not rated for higher temperatures. Caveat emptor.

3) You mentioned 'high conductivity', but are you sure? Have you measured the conductivity? Magmeters require a minimum conductivity to operate. Distillation can reduce water's conductivity.

4) Magmeters measure velocity, so you need a reasonable velocity to get a reasonable measurement. That means you might need to reduce pipe diameter in order to get a reasonable velocity for measurement purposes.

 

[Kdolph]

Thanks danw2

My answers:

3) I'm sure. The purge is for lowering the conductivity. This is a lot more than 10 us/cm
4) At 20 m3/hr I have 1 m/s. Is it enough?

 

[danw2]

>3) conductivity more than 10 us/cm
That's good. Most mags have a minimum of 5uS.

>4) At 20 m3/hr I have 1 m/s. Is it enough?
Yes. Minimum velocity is generally 0.3m/s.